French has two future tenses -- the futur proche and the futur simple.
The futur proche is formed with the auxiliary aller which is followed by an infinitive (Je vais partir. 'I'm going to leave').
The futur simple doesn't have an auxiliary.
Instead, the infinitive form becomes the stem to which future endings are added (Je partirai. 'I will leave').
For more details on the formation of these two future tenses, including irregular forms, see the related links:
future: regular, future: irregular, and futur proche.
futur proche vs. futur simple
The two tenses are virtually interachangeable in most contexts, especially in spoken French.
So what is the difference? In general, the two tenses differ in their level of formality;
the future proche is used in more informal contexts and the simple future in more formal contexts.
Thus, the futur proche is primarily used in speech and less frequently in writing.
According to traditional grammars, the two tenses also differ in their relative distance to the present moment.
The future proche, also called the futur immédiat, typically refers to a time very close to the present moment, i.e, the near or immediate future.
The futur simple, on the other hand, is often used for events in the more distant future.
Because the futur simple is associated with distant future events, it often takes on a detached, objective quality making it the preferred tense for future events that represent general truths.

Qui vivra, verra.

Whoever will live, will see.

L'homme sera toujours l'homme.

Man will always be man.(Boys will be boys.)

The two tenses also indicate a difference in the speaker's perception of the future event.
The futur proche indicates that the speaker is relatively certain that the future event will actually happen.
In contrast, the futur simple indicates that the speaker is less certain of the future event coming to pass.
Let's imagine a context to make this distinction more clear.
Suppose that a very disturbed man has just climbed out onto the ledge of a skyscraper.
The man begins to lose his balance. A horrified onlooker sees the man beginning to teeter and screams:

Il va tomber!

He's going to fall!

Il tombera!

He will fall!

In this scenario, first sentence, the one in the near future, sounds more plausible in French and in English. Why?
Because when someone loses his balance, he will invariably fall.
In this case, the tense indicates both the immediacy of the future event as well as its inevitability in the mind of the speaker.
Of course, speakers are usually more certain about the immediate future and less certain about the distant future.
Does this mean that certainty and distance to the present always go together? Not exactly.
One can be certain about events in the distant future too.
For example, look at the following sentences in French and in English and try to imagine the different contexts in which they would be appropriate.

Je vais avoir un enfant!

I'm gonna have a baby!

J'aurai un enfant!

I will have a baby!

In the first sentence, the speaker is either pregnant or has just received confirmation from an adoption agency.
Whatever the case may be, the speaker knows for certain that she will become a parent and indicates the inevitability of the future event via the futur proche.
But note that even though the speaker is certain about the future event, the event is not likely to happen immediately.
In the second sentence, the speaker is stating her desire to either get pregnant or to adopt a child of her own.
In this sentence the time frame for the future event is left open--maybe she will get pregnant tomorrow, maybe she won't.
Now consider the following dialogue in which Tex's sister Rita discovers she's going to have a baby. Can you explain the choice of future tense?

Tammy: Yes, you will be a little tired maybe, but joy will give you energy. You will see.

Rita: Ma joie?! Je serai heureuse quand mon mari aura une vasectomie.

Rita: My joy?! I will be happy when my husband has a vasectomy.

differences between French and English
In general, the future tenses in French and in English are used similarly.
However, there is one major difference in future tense usage between the two languages.
French requires the future tense after certain conjunctions where English usage calls for the present tense.

Joe-Bob: When Tex and Tammy come back, they will spend a few days in Opelousas.

Corey: Et quand ils reviendront à Austin, on fera la fête!

Corey: And when they return to Austin, we'll have a party!

In the French examples above, the subordinate clauses starting with dès que, aussitôt que, lorsque, and quand are in the simple future because the main clauses are in the future.
The action in the subordinate clause implies future tense since it will take place at around the same time as the action in the main clause.
Note the use of the present tense in the subordinate clause of the English translations.

'Futur simple' or 'futur proche'? Fill in the blank with the appropriate future tense of the verb indicated in parentheses.